Diagnostic Approach

When an IEM is possible, the clinical features help focus the approach to diagnosis. In the setting of acute illness, consider an IEM in parallel with more common disorders, even with a noninformative family history. Examine the blood and urine of patients with acute neurological deterioration for signs of acidosis, ketosis, hypoglycemia, and hyperammonemia (Tables 62.1 and 62.2). Also consider screening tests for abnormalities of amino or organic acids. Abnormal metabolites may not be present during stable periods or in samples obtained after the acute illness is over.

Table 62.1 Commonly Requested Tests for the Evaluation of a Patient Thought to Have an Inborn Error of Metabolism

CSF, Cerebrospinal fluid; IVA, isovaleric acidemia; MMA, methylmalonic acidemia; OXPHOS, oxidative phosphorylation; PA, propionic acidemia; PDHC, pyruvate dehydrogenase complex.

* Presence or absence of hypoglycemia can be a useful aid to differential diagnosis of disorders that lead to lactic acidemia.

† Hyperuricemia may lead to formation of nephrolithiasis, obstructive nephropathy, and gout.

Certain situations require an analysis of cerebrospinal fluid (CSF) (Jones et al., 2006). For example, determination of CSF levels of biogenic monoamines and γ-aminobutyric acid (GABA) may be diagnostic in severe neonatal/infantile epileptic encephalopathy due to neurotransmitter defects (Table 62.3). Such defects are suggested in infants and children with (fluctuating) extrapyramidal disorders, in particular parkinsonism, dystonia or more general “athetoid cerebral palsy,” and vegetative disturbances. Other diagnoses for which CSF testing may be informative include nonketotic hyperglycinemia, GABA transaminase deficiency, and homocarnosinosis. Proper collection and handling of the CSF is important to avoid spurious results (Hyland, 2003).

Neurological deterioration is a characteristic feature of acute intoxication disorders (e.g., certain aminoacidopathies [maple syrup urine disease, MSUD], organic acidemias [methylmalonic acidemia, MMA; propionic acidemia, PA; isovaleric acidemia, IVA; multiple carboxylase deficiency, MCD], and the urea cycle defects). Abnormal urine odor is present in diseases associated with the excretion of volatile metabolites (maple syrup in MSUD; sweaty feet in IVA and glutaric acidemia type II). Isolated seizures are often the initial features of vitamin-responsive disorders (e.g., defects of pyridoxine and folinic acid metabolism, biotin-responsive MCD), and are a prominent feature in nonketotic hyperglycinemia, sulfite oxidase deficiency, and congenital malabsorption of magnesium. Congenital lactic acidosis and central hypotonia are features of deficiencies of pyruvate carboxylase (PC) and pyruvate dehydrogenase (PDH) and disorders of the Krebs cycle and mitochondrial respiratory chain (Pithukpakorn, 2005). Recurrent hypoglycemia typically occurs in the glycogen storage disorders (GSD) affecting the liver and in defects of FAO (consequent to overutilization); these diseases are also often associated with signs of cardiac involvement (cardiomyopathy, arrhythmias). In the mitochondrial FAO defects, urinary ketone concentrations are low, and the ratio of 3-hydroxybutyrate/acetoacetate is usually less than 2.0. Analysis of plasma acylcarnitine and urinary acylglycine profile are particularly helpful in these cases. Renal tubular acidosis associated with HCO₃⁻ loss in urine can be encountered in PC deficiency, MMA, carbonic anhydrase deficiency, carnitine palmitoyltransferase I (CPT-I) deficiency, and cystinosis.

Ophthalmological examination often provides a clue to the diagnosis of IEM. Vertical supranuclear ophthalmoplegia occurs in Niemann-Pick disease type C, and saccadic initiation failure and defective optokinetic nystagmus occurs in Gaucher disease type III. Kayser-Fleisher rings (orange or greenish deposits around the limbus of the cornea due to copper deposition within the Descemet membrane) are a feature of Wilson disease. Table 62.4 shows additional ophthalmological findings characteristic of IEM.

Table 62.4 Ophthalmologic Findings Associated with Inborn Errors of Metabolism

Hepatosplenomegaly and other signs of storage (e.g., coarse facies, nonimmune hydrops fetalis, dysostosis multiplex) occur with the lysosomal disorders. Liver dysfunction or hepatomegaly (or both) usually occur in defects of carbohydrate metabolism (galactosemia and hereditary fructose intolerance, GSDs, [particularly Pompe disease], and bile acid synthesis), and in tyrosinemia and CDG. Unconjugated hyperbilirubinemia associated with liver dysfunction or hemolysis in infancy may lead to permanent brain damage due to kernicterus.

Cardiomyopathy may develop in IEM associated with infiltrative (storage) disorders and defects of energy metabolism. The presence of hepatomegaly and other signs of systemic involvement (e.g., cataracts, coarse facies, dysostosis multiplex) may suggest storage disorders of glycosaminoglycans. Defects of energy metabolism may be associated with acute or chronic encephalopathy, hepatic dysfunction, and several biochemical abnormalities (e.g., hypoglycemia, lactic acidosis ± ketosis, and elevated liver transaminase levels).

Some disorders may have both a young and a later age of onset or follow an atypical course. Different allelic mutations with partial enzyme deficiencies or organ-specific expression may explain these differences. Examples include acid maltase deficiency (muscle weakness and respiratory problems in the absence of cardiomyopathy), FAO defects (myoglobinuria and rhabdomyolysis after extreme exercise), X-linked adrenomyeloneuropathy (spastic paraparesis secondary to demyelination of the spinal cord and peripheral nerves), glycogen brancher enzyme deficiency (adult polyglucosan body disease with progressive upper and lower motor neuron disease, sensory loss, neurogenic bladder, and dementia), and acute intermittent porphyria (abdominal pain, psychosis).

Tandem mass spectrometry (TMS) analysis of blood spots on filter paper is an effective means of screening for some defects of amino and organic acid metabolism and FAO defects (Pasquali et al., 2006) (Table 62.5). Methods for screening of lysosomal storage disorders by TMS have also been recently introduced (Matern, 2008). Metabolic profiling of infant urine through comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry for the diagnosis of organic acidurias and biomarker discovery is also an option (Kouremenos et al., 2009). The main advantages of TMS over previous methods of newborn screening are improved accuracy, sensitivity and specificity, and suitability for cost-effective multi-IEM screening. Furthermore, its use avoids the need for potentially harmful procedures (i.e., fasting, substrate loading).

Table 62.5 Metabolic Disorders Detected Through Tandem Mass Spectrometry

* The abbreviations for fatty acid oxidation products and organic acid intermediates (e.g., C5, C160H, C18:1, etc.) refer to carnitine esters of aliphatic monocarboxylic acids, with chain length indicated by the number adjacent to C, and the number of double bonds indicated by the number after the colon.

From Centers for Disease Control and Prevention, 2001. Using tandem mass spectrometry for metabolic disease screening among newborns. Recommendations and reports. MMWR Morb Mortal Wkly Rep 50, April 13, 2001.

Assessment of carnitine profile (total and esterified carnitine levels and urinary carnitine excretion patterns) may also prove useful when suspecting a primary or secondary carnitine deficiency. Carnitine plays an essential role in the transfer of long-chain fatty acids across the inner mitochondrial membrane, in the detoxification of acyl moieties, and in the maintenance of free coenzyme A levels. Dietary sources provide carnitine. Primary carnitine deficiency (due to defective transport protein encoded by the OCTN2 gene) leads to increased urinary loss and cardiac and skeletal muscle disease. Secondary carnitine deficiency occurs in several IEM and may be partially responsive to oral carnitine supplementation. Table 62.6 shows the differential diagnosis of disorders involving carnitine metabolism.

Histological examination of appropriate tissue samples can provide clues to the nature of the storage materials found in certain IEM (e.g., lysosomal disorders) (Warren and Alroy, 2000). When performing a skin biopsy, it is advisable to obtain samples for microscopic examination and tissue culture. These are useful as source material for subsequent biochemical or molecular (genetic) testing. Disorders of amino and organic acid metabolism are not associated with deposition of storage material, and histological findings are nonspecific. For this group of disorders, use skin fibroblasts for confirmatory diagnosis by enzymatic assays. For some disorders, biochemical testing is not accurate for carrier detection, because residual enzyme activity in a significant proportion of carriers overlaps with values obtained from the general population. In certain IEM, molecular assays may be available for diagnostic purposes and carrier testing. Microscopic examination of scalp hair can provide important diagnostic information in some cases, such as in Menkes syndrome (pili torti) (Smith et al., 2005).

The GeneTests website (www.genetests.org) lists diagnostic laboratories performing specialized genetic tests. Careful attention to sample requirements and shipping and handling considerations is imperative. Clinical information is required with the specimen to receive expert advice in patient evaluation. In cases with an established diagnosis, detailed information may be obtained from several websites, including Online Mendelian Inheritance in Man (OMIM [www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM]), GeneClinics (www.geneclinics.org), and the National Organization for Rare Disorders (NORD [www.rarediseases.org]). Several support and patient advocacy groups provide information about community resources and ongoing clinical trials.

Mutation Analysis in the Diagnosis of Inborn Errors of Metabolism

Molecular genetic techniques offer an alternative means for the diagnostic confirmation of IEM. This is particularly true for diseases with a founder effect—common mutations in which one or a few alleles account for a significant proportion of cases (e.g., Finnish and Jewish “heritage” diseases). In diseases with a known causal mutation, testing of other family members permits accurate carrier identification. Deoxyribonucleic acid (DNA) analysis for prenatal diagnosis provides rapid diagnosis because the use of chorionic villi or amniocytes obviates the need for culture (although in practice, most laboratories insist on subsequent confirmatory testing of cultured cells and the need to exclude maternal cell contamination). Caution should be applied in assignment of causality, particularly for novel mutations or sequence alterations wherein functional impact has not been established.

Examples of disorders for which DNA testing has proven useful include medium-chain acyldehydrogenase [MCAD] deficiency, myophosphorylase deficiency (McArdle disease), and Gaucher disease (Gregersen et al., 2004; Madonna et al., 2002; Maire, 2001). Among patients with MCAD deficiency of northwestern European descent, 80% are homozygous for a single missense mutation (A985G), and 17% carry this mutation in combination with another less common defect. This finding has improved the reliability of MCAD carrier identification and diagnosis, particularly of siblings who may be affected but asymptomatic at the time of family screening. Certain metabolic disorders (e.g., Tay-Sachs disease, Gaucher disease, Niemann-Pick disease type A, and Canavan disease) have an increased prevalence among individuals of Ashkenazi Jewish ancestry (i.e., of Central and Eastern European descent). A limited number of “common” mutations in this population cause the disease. This has enabled targeted screening for appropriate counseling prior to marriage. In hereditary fructose intolerance and dihydropyrimidinase deficiency, screening for the disease mutation in blood obviates the need for liver biopsy. However, in mitochondrial disorders that cause myopathy, biopsy of a weak muscle is still required, because expression of the mutant gene may not occur in other tissues. Fresh muscle tissue is also the preferred sample for studies of respiratory chain function.

Inborn Errors of Metabolism Associated with Abnormal Brain Development and Encephaloclastic Lesions

Several metabolic disorders can cause a disruption of the normal sequence of brain development and lead to encephalocele, dysgenic corpus callosum, and neuronal migration defects (Hennekam, 2005; Jeng et al., 2001; Nissenkorn et al., 2001) (Tables 62.7 and 62.8). For instance, cystic necrosis of white matter, with or without basal ganglia involvement, occurs in PDH, PC, and molybdenum cofactor deficiencies. Nonsyndromic congenital microcephaly has been associated with maternal PKU, phosphoglycerate dehydrogenase deficiency, and 2-ketoglutaric aciduria (the cause of Amish lethal microcephaly). The proposed mechanisms to explain abnormal brain development and encephaloclastic lesions (such as porencephalic cysts) in IEM include the production of a toxic or energy-deficient intrauterine milieu, modification of the content and function of membranes, and disturbance of the normal expression of intrauterine genes responsible for neurulation and neuronal migration (Prasad et al., 2009).

Neuroimaging in the Diagnosis and Management of Neurometabolic Brain Diseases

Magnetic resonance imaging (MRI) has been widely used to obtain brain anatomical data in patients with static and progressive encephalopathy. Advanced imaging techniques such as MR spectroscopy (MRS) and diffusion-weighted imaging (DWI) provide additional information relating to brain biochemistry and cell viability and point to the diagnosis of a neurometabolic disease (Cheon et al., 2002; Faerber and Poussaint, 2002; Kaye, 2001). MRS and DWI are quantitative techniques that may prove useful in assessing severity of the underlying pathological changes and potential response to treatment.

White-matter signal abnormalities consistent with leukodystrophy can be found in several metabolic disorders such as Krabbe disease, metachromatic leukodystrophy (MLD), X-linked adrenoleukodystrophy, and Canavan disease. In addition to signal hyperintensities noted on T2-weighted imaging, special features may provide additional clues to the diagnosis. Enlarged perivascular spaces or small cysts are characteristically noted in the mucopolysaccharidoses and Lowe syndrome. Gray-matter lesions may be found in Zellweger syndrome and other peroxisomal disorders. In glutaric aciduria type 1, imaging may show selective frontotemporal atrophy, especially involving subcortical white matter, with prominent extraaxial CSF collection, widening of the sylvian fissure with poor opercularization (“bat wing” appearance), and in some cases, subdural hemorrhage (Oguz et al., 2005). The presence of T2 hyperintense lesions in the subcortical cerebral white matter, basal ganglia, and dentate nuclei, together with cerebellar atrophy, is considered pathognomonic of l-2-hydroxyglutaric aciduria (Steenweg et al., 2009).

MRS has enabled definition of the creatine deficiency syndromes, a newly discovered group of disorders causing mental retardation and other neurological problems (i.e., extrapyramidal movement abnormalities, hypotonia). The common feature is severe depletion of brain creatine/phosphocreatine. With brain MRS, elevated brain levels of N-acetylaspartate (NAA) are seen in patients with Canavan disease, whereas patients with mitochondrial defects, defects of gluconeogenesis, and biotin-responsive multiple carboxylase deficiency may show elevated brain lactate levels. In MLD, MRS reveals decreased NAA and increased choline and myoinositol compatible with axonal loss, dysmyelination, and gliosis. A prominent signal at 8.3 ppm in gray-and white-matter brain regions has been described in patients with adenylosuccinate lyase deficiency (Henneke et al., 2010). In patients with MSUD and acute metabolic decompensation, MRS demonstrates decreased NAA, methyl resonance of branched chain amino acids at 0.9 ppm, and elevated lactate (Cakmakci et al., 2009). In patients with X-linked adrenoleukodystrophy, the NAA/choline ratio (at a cutoff point of 0.5) has been shown to predict disease progression in six cases with cerebral disease (Eichler et al., 2002). Multislice proton MRS imaging was useful in identifying impending or beginning degeneration in white matter that still appeared normal on conventional MRI.

Imminent Death Prior to Diagnosis in a Child with a Suspected Inborn Error of Metabolism

Obtain samples needed for diagnosis when a child develops acute fatal metabolic decompensation and in cases of sudden and unexpected death (Olpin, 2004). A correct diagnosis may help families as they cope with their loss and enables appropriate counseling and prenatal diagnosis for subsequent pregnancies. Freeze plasma (separated from whole blood) and urine (CDC, 2003). Blood and bile (obtained by direct puncture of the gall bladder) specimens can also be collected, spotted on filter paper (two circles for each, about 25 µL), and dried before being sent to the laboratory. Obtain a skin sample under sterile technique (use alcohol swabs and not iodine, which interferes with cell growth), and store it at room temperature in tissue culture medium. When suspecting a storage disorder, obtain a small snip of skin and place it in glutaraldehyde for subsequent electron microscopic studies.

Management Considerations

The appropriate management of IEM depends upon the particular metabolic and/or molecular derangement. Therapeutic strategies may include one or more of the following approaches: (1) substrate reduction by dietary manipulation or precursor synthesis inhibition, (2) removal (or enhanced clearance) of the toxic metabolites, (3) replenishment of depleted metabolites or cofactor supplementation (or both), (4) enzyme (replacement or enhancement) therapy, and (5) cellular replacement (e.g., bone marrow, liver, heart, or kidney transplantation) (Chakrapani and Wraith, 2002; Leonard, 2004; Wilcken, 2003). Previous active exploration of gene therapy ended with the unexpected death of a study subject with ornithine transcarbamylase (OTC) deficiency. Reexamination of this approach led to demands for greater caution in future clinical trials (Hsich et al., 2002; Sands and Davidson, 2006). Meanwhile, gene therapy experiments are ongoing in various animal (mouse) models of disease (Koeberl et al., 2009). Other approaches under consideration are liver repopulation (hepatocyte infusion), chaperone-mediated therapy for diseases associated with residual enzyme activity, and the transplantation of stem cells with directed differentiation along specific lines (Desnick, 2004; Grompe, 2002; Kawashita et al., 2005). The rationale for the use of pharmacological chaperones is the demonstration that certain small molecules may rescue misfolded proteins in the endoplasmic reticulum and restore functional conformation (Gregersen et al., 2001; Pastores and Sathe, 2006). Certain drugs have also been shown in vitro to have the potential of reading through stop codons and may induce the synthesis of a full-length protein. A short-term study of gentamicin in a patient with McArdle disease and the R50X mutation failed to show modification of muscle metabolism (Schroers et al., 2006); these observations may be influenced by the short duration of the study or underlying gene defect. Clearly, more studies are required to examine the potential of this approach. Bezafibrate has been shown to increase carnitine palmitoyl transferase II (CPT2) messenger ribonucleic acid (mRNA), with normalization of enzyme activity in CPT2-deficient cultured fibroblasts and myoblasts. In a pilot trial including six patients treated with bezafibrate, FAO concentrations normalized in all patients, palmitoyl-l-carnitine oxidation increased, episodes of rhabdomyolysis were reduced, and quality of life improved without adverse effects (Bonnefont et al., 2009). These advances have changed the attitude about treatment for patients with IEM away from nihilism and hopelessness toward cautious optimism.

Each patient requires an individualized approach; some disorders require more than one management option. The clinical response to most treatment plans may vary, and residual disturbances are common. Patients may remain at risk for metabolic decompensation when stressed by infection, trauma, or surgery. Reducing energy expenditure and promoting anabolism are immediate management goals. Emergency measures may prevent further deterioration, but most options are nutritionally incomplete, and extension beyond 48 hours without dietary review is wrong. In most situations, provision of symptomatic treatment requires specialized care units with expertise in the specific disease.

Special diets require attention to caloric needs and balanced nutrition; this is particularly true for minerals and supplements. Diseases managed with dietary restriction include the aminoacidopathies: phenylketonuria (PKU), MSUD, and homocystinuria. Recent clinical studies suggest that treatment of PKU patients with sapropterin (a cofactor of phenylalanine hydroxylase) provides better disease control and increases dietary phenylalanine tolerance, allowing significant relaxation, or even discontinuation, of dietary phenylalanine restriction (Blau et al., 2009). In classic Refsum disease, reduction in dietary phytanate results in normalization of the biochemical and clinical phenotype.

Some disorders require alternative dietary sources. Administer medium-chain triglycerides as a lipid source to patients with very long-chain acyldehydrogenase [VLCAD] and long-chain acyldehydrogenase [LCHAD] deficiency. In Smith-Lemli-Opitz syndrome (3β-hydroxysterol-Δ7-reductase deficiency), the use of a high-cholesterol diet (± bile acids) improves growth and neurodevelopmental status, although clinical response is variable. In children with GSDs, carbohydrate supplements prevent hypoglycemia and suppress secondary metabolic derangements (e.g., hyperlipidemia, hyperuricemia). In the urea cycle disorders, arginine or citrulline supplements make up for compounds that are not synthesized secondary to the metabolic block.

Employ methods to enhance the excretion or detoxification of toxic metabolites when dietary manipulation fails to correct their accumulation. In patients with hyperammonemia, give sodium benzoate and sodium phenylbutyrate, which conjugate with glycine and glutamine to facilitate nitrogen excretion (Fig. 62.1). In IVA, treatment with oral glycine (500 mg/kg) conjugates the highly toxic isovaleric acid to form a harmless byproduct excreted in the urine. Administration of cysteamine to patients with cystinosis promotes the formation of cysteine, which is subsequently excreted in the urine. Carnitine supplementation given to patients with organic acidemia prevents carnitine deficiency secondary to the formation and renal excretion of acylcarnitine compounds. During acute metabolic decompensation, dialysis and hemofiltration facilitates the rapid clearance of toxic metabolites. These techniques are already a part of the treatment of MSUD, carbamoylphosphate synthetase (CPS) deficiency, and hyperammonemic or leucine encephalopathy (Daschner and Schaefer, 2002). In patients with urea cycle defects, hemodialysis should be considered at ammonia levels above 600 mol/L (1000 g/L). Administration of glycerol trioleate and glycerol trierucate (Lorenzo oil) to asymptomatic patients with X-linked ALD may modify the disease course (Moser et al., 2005).

Fig. 62.1 Alternative pathways to reduce accumulation of ammonia.

Novel therapeutic strategies for IEM use substrate synthesis inhibitors to block the production of toxic metabolites. In tyrosinemia type I (fumaroylacetoacetate hydrolase deficiency), NTBC (2-nitro-4-trifluoro-methylbenzoyl-1,3-cyclohexanedione) reduces the production of downstream metabolites of tyrosine degradation by inhibiting the enzyme, 4-hydroxyphenylpyruvate dioxygenase. This enzyme is involved in a reaction preceding the block (Fig. 62.2). In one study of NTBC in more than 300 patients with tyrosinemia type 1, 95% showed improvement of hepatic and kidney function (Grompe, 2001). In Gaucher disease type I, NB-DNJ (N-butyldeoxynojirimycin), miglustat administration leads to decreased liver and spleen volumes and a gradual but significant improvement in hematological parameters, with a decline in the levels of disease activity markers (Cox et al., 2000). NB-DNJ inhibits ceramide-specific glucosyltransferase, the first enzymatic step in glycosphingolipid (GSL) biosynthesis. Limiting substrate accumulation to a level that is sufficiently catabolizable by a mutant but partially active enzyme achieves metabolic homeostasis. Given this mechanism of action, miglustat may be potentially useful for other disorders of GSL metabolism (e.g., late-onset Tay-Sachs, Sandhoff disease, GM1-gangliosidosis, and Niemann-Pick disease type C [NPC]) (Pastores and Barnett, 2005).

Fig. 62.2 Substrate reduction therapy.

Unfortunately, the miglustat trial in patients with either late-onset Tay-Sachs or Gaucher disease type 3 on enzyme replacement therapy was unable to prove any measurable benefit (Schiffmann et al., 2008; Shapiro et al., 2009). In patients with NPC, treatment with miglustat resulted in improvements in saccadic eye movements and swallowing, but impact on survival and eventual neurological prognosis appeared limited (Patterson et al., 2007).

Replenishing depleted substrates may partially correct the underlying defect in some IEM. In carnitine transport defects, the use of carnitine results in resolution of cardiomyopathy and prevention of further episodes of hypoketotic hypoglycemia. In other disorders, the production or binding affinity of a cofactor required for enzyme activity is impaired. Administration of pharmacological doses of the required supplement corrects the defect. Biotin, in some cases of PA and when given to children with biotinidase or holocarboxylase deficiency, leads to good clinical outcomes, except in the most severe forms with neonatal onset (Wolf, 2002). Vitamin B₁₂ given for late-onset forms of MMA due to defects of adenosylcobalamin metabolism leads to a sustained decrease of toxic metabolites and a favorable developmental prognosis. Suppression of gut microbial propionate production (using metronidazole to inhibit anaerobic colonic flora) and dietary protein restriction are complementary approaches provided to children with MMA.

Additional examples of pharmacological correction include the use of tetrahydrobiopterin (BH4) to treat disorders of biopterin synthesis, rare variants of the hyperphenylalaninemia syndrome (Lambruschini et al., 2005). Although BH4 reduces elevated plasma phenylalanine levels by its action on liver phenylalanine hydroxylase, penetration of the blood-brain barrier is minimal. Only those with a peripheral-type defect respond. Tetrahydrofolate prevents demyelination in children with folate deficiency and dihydropteridine reductase deficiency. Assess the responsiveness to cofactor administration by controlled enzyme assays involving the patient’s cultured skin fibroblasts. Table 62.9 lists the cofactors used in various metabolic disorders and their recommended dosages.

Table 62.9 Cofactors Used in the Management of Various Inborn Errors of Metabolism

IVA, Isovaleric acidemia; MMA, methylmalonic acidemia; MTHFR, methylene tetrahydrofolate reductase; PA, propionic acidemia.

* Dose is up to 150 mg/kg/day.

† Avoidance of fasting is critical; in emergency cases, where anorexia or vomiting precludes oral intake institute intravenous glucose administration.

Enzyme replacement therapy (ERT) reverses the hematological and visceral manifestations of Gaucher disease and may prevent or stabilize disease-related bone complications. This approach is also a consideration for the treatment of LSD due to single enzyme deficiencies. The relevant enzymes, produced from genetically manipulated mammalian cells in culture and subsequently modified, expose the appropriate sugar residues to facilitate targeted cell uptake. Once purified, management requires regular intravenous infusions of the recombinant enzyme. Beneficial effects have been noted in patients with Fabry disease, mucopolysaccharidosis (MPS) type I (Hurler-Scheie syndrome), MPS II (the mild variant of Hunter syndrome), MPS type VI (Maroteaux-Lamy syndrome), and GSD II (Pompe disease) (Pastores and Barnett, 2005). Enzyme therapy is also a consideration for other LSDs including Niemann-Pick disease.

Metabolic correction through cellular replacement by bone marrow transplantation (BMT)/hematopoietic stem cell transplantation (HSCT) has been performed in patients with LSD (e.g., MPS I, Gaucher disease type III) (Peters et al., 2003). In X-linked adrenoleukodystrophy, BMT has resulted in prolonged remission with reversal of MRI abnormalities and stabilization or improvement of motor function (Moser, 2006; Peters et al., 2004). Although BMT has altered the natural course of these diseases, donor limitation issues, procedural risks, and long-term care considerations (e.g., immunosuppression) exist. Advances in the methods of conditioning (e.g., nonmyeloablative procedures) prior to BMT and utilization of umbilical cord blood have addressed some of these concerns. Patients with IEM who may be suitable candidates for BMT should have serial follow-up visits incorporating neuropsychological and neuroradiological studies to time interventions prior to significant disease progression. This allows an outcome with minimal neurological sequelae. BMT is not appropriate for disorders characterized by rapid neurodegeneration, such as MPS II (severe Hunter syndrome) and MPS III (Sanfilippo syndrome).

Organ transplantation may be appropriate for disorders in which the metabolic defect is confined to the liver (e.g., Crigler-Najjar syndrome, hyperoxaluria type I) or leads to single organ failure (e.g., end-stage renal insufficiency in Fabry disease and hyperoxaluria type I, and liver failure in OTC deficiency, tyrosinemia, and GSD-IV) (Dhawan et al., 2005; Inderbitzin et al., 2005). In patients with tyrosinemia who do not respond to NTBC therapy or have evidence of hepatic malignancy, orthotopic liver transplantation is the treatment of choice. Microchimerism (the migration of donor-derived cells from the allograft) has occurred in a few patients following liver transplantation. However, this phenomenon is probably not sufficient to correct the systemic metabolic defect.

Symptomatic treatment remains a vital component of patient care. Indeed, several palliative measures improve quality of life and reduce the incidence and severity of disease-related complications. For instance, l-dopa improves motor function in patients with tyrosine hydroxylase deficiency. DDAVP (d-deamino-arginine-vasopressin) reduces the tendency for abnormal bleeding during surgery of patients with GSD type 1A (von Gierke disease). Granulocyte colony-stimulating factor (G-CSF) administered to patients with GSD IB and neutropenia minimizes the risk of recurrent bacterial infection and gastrointestinal tract ulceration. Corticosteroid and mineralocorticoid replacement are essential in patients with ALD and adrenal insufficiency.

Patients with certain metabolic disorders necessitate special considerations for anesthesia and surgery. For instance, instability of the atlantoaxial joint and upper airway obstructive disease in patients with mucopolysaccharidosis may lead to problems during induction and extubation. Hypoglycemia must be avoided in patients with fatty acid oxidation defects, glycogen storage disorders, and disorders of gluconeogenesis. This may require careful planning in terms of the time the procedure is undertaken and maintenance of euglycemia through intravenous administration of 10% glucose. Surgery and anesthesia may induce a metabolic crisis in patients with Refsum disease by mobilization of phytanic acid in fat stores.

In addition to dealing with the medical problems of an affected child, provide genetic counseling. The inheritance of approximately 90% of IEMs is autosomal recessive. Of the remaining 10%, approximately two-thirds are X-linked and one-third autosomal dominant traits. Prenatal diagnosis is available for most IEM. A study that looked at reproductive decisions made by parents of children with IEM noted that 56% were receptive to future prenatal diagnosis, and 41% would choose to take measures to prevent another affected pregnancy (Read, 2002). The study also found that parents of children with IEM had higher scores on a stress index, lower scores on an adaptive behavior scale, and fewer persons in their social support network. Furthermore, the parents expressed greater worry about their child’s future and perceived difficulty in meeting the child’s extra care needs. These observations underscore the importance of early intervention, supportive care, and appropriate genetic counseling.

Adolescent with an Inborn Error of Metabolism and Transition to Adulthood

Because of early diagnosis and intervention, many affected children have achieved longer survival. The care received as a child greatly influences the overall prognosis and quality of life for these individuals in adolescence and adulthood. An important goal is for societal integration and life fulfillment, which may be possible in certain cases.

In most instances, the provision of patient care has been by pediatricians and metabolic specialists. The majority of physicians caring for adults are not prepared to assume the care of these patients as they grow older. Familiarity with the natural history of the disease may lead to anticipatory guidance and appropriate monitoring, with early intervention at the first sign of trouble (Enns and Packman, 2002). Several examples illustrate this. Hepatic adenomas, which may become malignant, develop in the second and third decade of life in patients with GSD IA. Patients with tyrosinemia are also at risk for hepatocellular carcinoma; this requires monitoring by serial α-fetoprotein measurements and liver imaging.

Other late complications include acute and chronic recurrent pancreatitis which occurs in association with the hyperlipidemias, disorders of branched-chain amino acid degradation, homocystinuria, and acute intermittent porphyria (Simon et al., 2001). Cholelithiasis occurs in patients with metachromatic leukodystrophy and Gaucher disease, and in disorders of bile formation and biliary transport. Atherosclerosis and thromboembolism are potential causes of morbidity in homocystinuria and methylenetetrahydrofolate reductase (MTHFR) deficiency. Cardiomyopathy and retinopathy may complicate LCHAD. Renal insufficiency or failure may develop in patients with cystinosis, Fabry disease, GSD I, and MMA. Metabolic stroke with bilateral globus pallidus abnormalities and extrapyramidal signs may be a sequela of metabolic decompensation in MMA.

As most IEM affect multiple systems, the involvement of a multidisciplinary team with central coordination by a primary physician is essential. Patients and family members usually appreciate being included in the decision-making process. These moments of interaction provide an opportunity to assess the family’s understanding of the disease and its management as well as their coping mechanisms.

Direct your efforts to ensuring that the child reaches his or her maximum potential. Educational programs for the affected child must be adapted to developmental level and cognitive strengths to minimize frustration and associated behavioral problems. It is important to prepare for development of increasing handicap and take appropriate steps to facilitate individual performance during activities of daily living. Self-care, communications skills, and mobility issues require special attention.

All individuals become increasingly self-conscious of their body image during puberty. Enhance self-esteem in adolescents who feel stigmatized by their physical appearance, particularly when the IEM is associated with facial dysmorphic features and skeletal deformities. Dysarthria impedes communication, and disturbances of bowel and bladder continence undermine self-confidence and social interaction. Some disorders are associated with delayed puberty (GSD IA, galactosemia, and CDG-1a), or premature ovarian failure (hypergonadotropic hypogonadism). Osteoporosis is often underrecognized as an associated condition of IEM. This most often occurs in disorders causing poor mobility due to cognitive or neuromuscular impairment or those characterized by chronic acidosis and renal insufficiency. When dietary regimens are required for disease control, expect the adolescent to manage the diet without parental assistance. Peer pressure may lead to noncompliance, and the patient must understand the potential implications of this course of action.

Pregnancy is a critical time, requiring measures to ensure a good maternal-fetal outcome (Preece and Green, 2002). Women of childbearing age with an IEM require disease control before conception. Close follow-up is required during labor, delivery, and the postpartum period. Children born to women with poorly controlled PKU are at risk for microcephaly (70%), attention deficit disorder and mental retardation (>90%), intrauterine growth retardation (40%), and congenital heart disease (12%). Risk to the fetus correlates with maternal phenylalanine levels. The achievement of a maternal blood phenylalanine concentration of less than 10 mg/dL (600 mM) by 8 to 10 weeks’ gestation and maintaining that concentration throughout pregnancy provides the optimal outcome.

Women with homocystinuria (cystathionine β-synthase deficiency) may have an increased risk for spontaneous abortion and preeclampsia. Pregnancy may exacerbate the cutaneous lesions of porphyria cutanea tarda during the first trimester. Women who are carriers of OTC deficiency may develop a hyperammonemic encephalopathy during the postpartum period. Postpartum metabolic decompensation also occurs in MSUD.

Lysinuric protein intolerance (LPI) is associated with increased risk of anemia, toxemia, and intrauterine growth retardation during pregnancy, and bleeding complications during delivery. Children of mothers with LPI generally develop normally. Attention to maternal protein intake and control of hyperammonemia and other problems during pregnancy are essential (Tanner et al., 2006).

Psychiatric symptoms are primary features of some disorders or may develop secondary to metabolic decompensation (Gray et al., 2000). For instance, behavioral changes (e.g., agitation, delirium) occur in individuals with X-linked ALD, late-onset GM2 gangliosidosis, metachromatic leukodystrophy, urea cycle defects, and porphyria. Cognitive and behavioral problems also occur in children with PKU, especially in those who are not compliant with dietary restriction. Children subjected to BMT are at risk for neuropsychological complications secondary to chemotherapy and irradiation.

Animal Models of Human Inborn Errors of Metabolism

Several spontaneous animal models of human IEM exist, but successful breeding is uncommon. Over the last decade, recombinant genetic techniques have enabled the generation of animal (primarily mouse) models. In a genotype-driven approach, homologous recombination in embryonic stem cells incorporating a null allele accomplishes the generation of mouse (knockout) models. Alternatively, introducing single point mutations result in mice with partial rather than complete deficiencies. A recent example is that of a homozygous knockout mouse which lacked branched-chain ketoacid dehydrogenase (BCKAD) activity and metabolic derangements leading to neonatal lethality (Homanics et al., 2006). Transgenic expression of the human BCKAD cDNA in the liver of this knockout model of MSUD produced a mouse with an intermediate phenotype; BCKAD activity was 5% to 6% of normal, which was sufficient to allow survival but with elevated plasma branched-chain amino acid levels. The currently available animal models have proven useful in investigations of the natural history of the disease and in the preclinical testing of various drugs. In some cases, these investigations have provided the rationale for further clinical trials in humans. Insights into potential disease mechanisms have also come from several animal studies. For instance, transgenic mice overexpressing glycogen synthase on a background of α-glucosidase deficiency show structurally abnormal polysaccharides similar to those observed in patients with Lafora disease and glycogenoses IV and VII (Raben et al., 2001). Although similarities may exist between affected mice and men, species-specific differences in disease expression result from alternative pathways of substrate processing. These considerations highlight the need for caution in carrying over observations made in the animal models to humans.

Lysosomal Storage Disorders

The lysosome is a membrane-bounded intracytoplasmic vacuole that contains enzymes required for the degradation of complex lipids, proteins, and nucleotides. Its acidic milieu (pH ≈ 5.4) is required for optimal activity of the contained hydrolytic enzymes and their cofactors and activators. Table 62.11 describes more than 40 different LSDs, with an estimated combined incidence of 1 in 5000 to 8000.

H.G. Hers first proposed the LSD concept in 1963, based on the detection of glycogen-filled vesicles of lysosomal origin in cells obtained from a patient with Pompe disease. Progressive lysosomal storage of incompletely metabolized substrates occurs either because of primary hydrolase deficiency (e.g., Fabry disease), deficiency of a protective protein that aids in the lysosomal targeting and prevention of premature degradation of enzymes (e.g., galactosialidosis), or the absence of an “activator protein” necessary for enzyme-substrate interaction and degradation (e.g., AB variant of GM2 gangliosidosis). Additional disease mechanisms include abnormal protein/enzyme processing in the endoplasmic reticulum, defects of posttranslational modification preventing functional enzyme maturation (e.g., multiple sulfatase deficiency), failure to attach the appropriate targeting signals (e.g., mannose-6-phosphate) in the Golgi apparatus (e.g., mucolipidosis II), and defective removal or transport of the substrate from lysosomes (e.g., Niemann-Pick disease type C and sialic acid storage disease) (Garver and Heidenreich, 2002; Gopaul and Crook, 2006). Certain enzymes are targeted to the lysosome via alternative routes; for example, glucocerebrosidase relies on LIMP-2 (Reczek et al., 2007). Recently, mutations in LIMP-2 have been identified in patients with focal glomerulosclerosis and progressive myoclonus epilepsy associated with storage material in the brain (Berkovic et al., 2008). In Danon disease, defects of lysosomal-associated membrane protein 2 (LAMP2) result in abnormalities in endocytosis, vesicle fusion, and the processing of autophagic elements (Sugie et al., 2002). LAMP2 is an integral membrane protein of endosomes and lysosomes. Other syndromes of intracellular vesicle damage that cause abnormal lysosomal formation and storage are Hermansky-Pudlak syndrome and Chédiak-Higashi syndrome.

Clinically the LSDs are a heterogeneous group of disorders involving multiple organ systems. The clinical features reflect the cellular sites of substrate storage and resultant organ dysfunction. There is incomplete understanding of disease pathogenesis, but there is increasing evidence for various mechanisms such as aberrant inflammation, induction of apoptosis, and defects of autophagy having a contributory role (Ballabio and Gieselmann, 2009).

In rapidly progressive forms, the onset of clinical features begins in the newborn or in early infancy. With later-onset forms, the initial features are delayed until adolescence or adult life, and the course can vary from acute to chronic. Acute and subacute courses are usually associated with primary central nervous system (CNS) involvement, developmental delay, and mental retardation.

Unlike the small-molecule diseases, the characteristic clinical features of LSDs are either a subacute or a chronic encephalopathy. Myoclonic seizures occur in the following disorders: fucosidosis, Gaucher disease types II and III, GM2 gangliosidosis, Schindler disease (α-N-acetylgalactosaminidase deficiency), and sialidosis type 1. Some LSDs do not have primary CNS involvement (e.g., Fabry disease, Gaucher disease type I, MPS I [Scheie syndrome], MPS IV [Morquio syndrome], mild MPS VI [Maroteaux-Lamy syndrome], and Niemann-Pick disease type B).

Defects in the enzyme cofactor/activator required for complete substrate hydrolysis, rather than a primary enzyme defect, cause rare variants of sphingolipid storage disorders. Two categories of sphingolipid activators exist. One represents the GM2 activator and the other a group of four molecules (saposin A, B, C, and D) derived by proteolytic cleavage of a common precursor, prosaposin. The gene localization for prosaposin is chromosome 10. Deficiency of the GM2 activator results in the AB variant of GM2 gangliosidosis. Saposin B activates arylsulfatase A. Deficiency of saposin B gives rise to a variant of metachromatic leukodystrophy (MLD variant). Saposin C activates glucocerebrosidase and β-galactocerebrosidase. The clinical picture of deficiency is an atypical form of Gaucher disease because of its clinical overlap with the type III variant (subacute neuropathic Gaucher disease). Characteristic of disorders resulting from cofactor deficiencies are normal enzymatic activities in vitro when using the synthetic (artificial) substrate. Therefore, routine biochemical testing misses the diagnosis. Molecular analysis may reveal the presence of mutations in the relevant encoding genes.

The transmission of all LSDs is autosomal recessive, except for Danon disease, Fabry disease, and Hunter syndrome (MPS-II), which are X-linked recessive traits. Biochemical assays are generally available for prenatal diagnosis. Take care in the interpretation of certain enzyme assay results (e.g., arylsulfatase activity, galactocerebrosidase), because low values may be obtained in the presence of pseudo-deficiencies. Diagnostic confirmation is also available by molecular (DNA) testing. In families with a known causal mutation, molecular testing enables accurate assignment of carrier or affected status. Prenatal diagnosis is possible for almost all LSDs. In certain cases, preimplantation genetic diagnosis may also be possible (Tomi et al., 2006).

Neufeld and colleagues showed that exchanging media from fibroblasts with different disease gene mutations (MPS I and MPS II) results in the clearance of intracellular storage material. The metabolic cross-correction was due to secretion of the functional enzyme from one cell line followed by intracellular uptake by the deficient cells. These studies provided the rationale for treatment of the LSD by ERT. Today, achieving cellular correction in certain clinical LSD subtypes is through BMT and ERT. Other therapeutic strategies under consideration include the use of substrate synthesis inhibitors, chaperone-mediated agents (Fig. 62.3), neuronal stem cell transplantation, and gene therapy (Sands and Davidson, 2006).

Fig. 62.3 Chaperone-mediated enzyme enhancement therapy.

Peroxisomal Disorders

The peroxisome is an organelle involved in β-oxidation of very long-chain fatty acids (VLCFA), the synthesis of plasmalogen (an ether lipid) and bile acids, and oxidation of pipecolic, phytanic, and dicarboxylic acids (Fidaleo, 2009). The classification of peroxisomal disorders is generally by the presence of single or multiple enzyme deficiencies (Table 62.12). Estimates of their combined incidence are 1 in 25,000. The internalization of most peroxisomal matrix proteins is through one of two targeting sequences in a unique system allowing the importation of oligomerized proteins through a specific shuttle involving a receptor and its cargo. Defects of these cellular mechanisms (involving peroxins encoded by PEX genes) lead to disruption of peroxisomal metabolic functions (Oglesbee, 2005). Peroxisomal disorders are mostly autosomal recessive traits, except for X-linked ALD, which is also the most prevalent clinical type (1 : 17,000).

Table 62.12 Peroxisomal Disorders*

Disorders Involving Small Molecules

Disorders of intermediary metabolism often result in the accumulation of compounds that cause acute progressive neurological disorders. The term defects of small molecules is used because the compounds that build up proximal to the metabolic block are often elevated in blood and CSF and sometimes excreted in the urine or potentially cleared by dialysis. Detection of these compounds in blood, CSF, and urine enables diagnosis. Serial measurements monitor the effectiveness of disease control. Treatment often requires elimination of the accumulating toxic compounds by dietary restriction or the provision of vitamins or cofactors. During episodes of acute decompensation, metabolic homeostasis may be rapidly achieved by exchange transfusion or preferably by peritoneal or hemodialysis. In neonates with unstable hemodynamics, continuous venovenous hemodialysis may be an option for extracorporal ammonia detoxification (Arbeiter et al., 2010). Continuous control necessitates the use of compounds that bind with the toxic molecules and facilitate alternative pathways of clearance. Included in this group are the aminoacidopathies, organic acidemias, and the urea cycle defects. Table 62.13 provides examples for each category and their estimated incidence.

Table 62.13 Disorders of Small Molecules and Energy Metabolism

MMA, Methylmalonic acidemia.

Miscellaneous Metabolic Disorders